Approximately 7,000 human immunodeficiency virus (HIV)-infected women give birth in the United States each year. Without treatment, about one-fourth of them transmit the virus to their children. The anti-HIV drug zidovudine (AZT), given to HIV-infected pregnant women before and during childbirth and to their infants after childbirth, reduces HIV transmission by as much as two-thirds. Treatment with AZT is now the standard of care in the U.S. for preventing HIV infection in infants. However, additional means are needed for the prevention of maternal to fetal transmission of HIV and other envelope viruses both in the U.S. and worldwide.